The present invention relates generally to terrain navigation-based on a passive signal reception. More particularly the invention relates to a non signal-emitting terrain navigation arrangement and a terrain navigation method. The invention also relates to a computer program and a computer readable medium.
In the last decades, navigation systems have been developed which facilitate the determination of a position dramatically compared to the earlier known methods. These navigation systems determine positions on basis of accurate radio signals received from a plurality of satellites (at least four), and are commonly referred to as global navigation satellite systems (GNSS:s). There exist several GNSS standards of which the most important examples are GPS (Global Positioning System, run by the U.S. Government), GLONASS (Global Navigation Satellite System, run by the Russian Federation Ministry of Defense) and the Galileo system (the European programme for global navigation services, which offers EGNOS (the European Geostationary Navigation Overlay Service), developed by a collaboration between the European Space Agency and the European Union).
Although the above-mentioned GNSS:s generally provide relatively accurate position information, GNSS-based navigation may become problematic if, for some reason, radio signals cannot be received from a sufficiently large number of satellites during an extended time, say 10 seconds or more. For example, such radio signal outage may occur if the line of sight vectors from the receiver to one or more of the satellites to which the receiver currently is locked are covered by obstacles. The radio signals from the satellites may also be jammed (intentionally or unintentionally) or be spoofed. Therefore., at least in military applications, it is often desirable to integrate the GNSS receiver with an INS (Inertial Measurement System) and a terrain navigation system, such that the systems may aid and support one another. In fact, a GNSS receiver and an INS complement each other quite well due to their fundamental differences.
A GNSS receiver and an INS measure different quantities. Accelerometers, found in inertial sensors, measure specific forces which are translated to a known coordinate frame with the aid of rate gyro derived measurements. Newton's laws of motion are then applied to provide velocity and position information. A GNSS is a radio navigation system. Hence, the GNSS receiver measures satellites-to-user ranges, and from these measurements, a user position and velocity can be derived. Furthermore, GNSS measurements have a long-term stability, however a noisy behavior. INS measurements on the other hand, are only short term stable; but are less noisy than GNSS receivers. Moreover, as mentioned above, since all GNSS:s are radio navigation systems, they are sensitive to external radio frequency disturbances; while an INS is not, since it is an autonomous system. Thus, an integration of a GNSS and an INS promises to combine the best of both worlds.
In terrain navigation, the position is estimated using a terrain database and a sensor for measuring the distance from a craft to the ground together with a sensor for measuring the craft's relative movement. In similarity with GNSS measurements, a terrain navigation system is long-term stabile, however relatively noisy.
Various solutions for combining a GNSS receiver with an INS are already known, for instance through the U.S. Pat. No. 6,449,559. This document describes a fully coupled positioning solution, wherein INS data is integrated with GPS data by means of a Kalman filter. Thereby, a positioning accuracy may be achieved, which is better than what would be possible to attain with a GPS receiver only.
In contrast, the U.S. Pat. No. 6,256,559 describes a purely GPS-based solution for determining a craft's altitude above the ground wherein both a direct signal and an earth-reflected signal from a satellite are measured. Thereby, a reliable passive altimeter is accomplished. However, only altitude measurements are discussed, not positioning.
Hence, the prior art both includes navigation solutions where a GNSS receiver and an INS are combined, and an altimeter solution where earth-reflected satellite signals are registered. However, there is yet no strictly passive navigation solution, which generates reliable position data also under conditions when signals from very few (i.e. less than four) satellites are available.